Just about have all the bugs worked out of the 85 12A.....I thought! I was downshifting from 5th to 4th too pass, when I completed the accelerater just stuck getting faster and faster. I turned the ignition off and pulled over to the side. Restarted and it worked fine. On other occasions when shifting at about 5000 RPMs, thenn engaging the cluch to shift the tach would shoot to 8000 RPMs then go back to normal as quick as it screwed up. Any clues? Thanks!

 

Spray some WD-40, or similar, on the pedal hinge and the cables that run to the carb. Work them back and forth and spray again.

The other prob...... well, not to be a smart-***, but are you backing your right foot off a little while shifting?

Does the motor rev or just the tach?

 

Maybe your secondaries are getting stuck open? Check that the spring is properly seated (metal solenoid case on right of carb, standing on passenger side). Careful of the gasket. Also spray carb cleaner on the arm/linkage below.